Hey,
My husband worked in child care and he suggests to have her wear undies all the time and take her to the toilet every half hour, straight after food and water (just like a puppy). Make them her special undies, she will be less likely to soil them when she figures out how special the are. He said if you need to put a nappy on (if your going out or have guests over) put the undies on top of her nappy (so she thinks she is wearing her special undies) and still take her to the toilet regularly.
hope that helps